If the grips control just one object, then override NewGeometry(). When
NewGeometry() is called, return new geometry calculated from the current
grip locations. This happens once at the end of a grip drag.

Resets location of all grips to original spots and cleans up stuff that
was created by dynamic dragging. This is required when dragging is
canceled or in the Copy command when grips are "copied". The override
should clean up dynamic workspace stuff.

Just before Rhino shades an object with grips on, it calls this method
to update the display meshes. Grips that modify surface or mesh objects
must override this function and modify the display meshes here.